It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the particulate filter and the selective reduction catalyst, thereby attaining lessening of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst, so that in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a urea water added position and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact exhaust emission control device as shown in FIG. 1 has been proposed by the applicant as JP 2008-196328A. In the exhaust emission control device of the prior application shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particles in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. A discharge end of the particulate filter 5 is connected to an entry end of the selective reduction catalyst 6 through an S-shaped communication passage 9 with a urea water addition means 10 at its upstream side such that the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is reversely curved back into the entry end of the adjacent selective reduction catalyst 6.
Arranged in the casing 7 and in front of the particulate filter 5 is an oxidation catalyst 11 for oxidization treatment of unburned fuel in the exhaust gas 3, and arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia reducing catalyst 12 for oxidization treatment of surplus ammonia.
With such construction being employed, particulates in the exhaust gas 3 are captured by the particulate filter 5. The urea water is added into the exhaust gas 3 upstream of the communication passage 9 by the urea water addition means 10 and is pyrolyzed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 3 is favorably reduced and depurated by the ammonia on the selective reduction catalyst 6. As a result, both the particulates and NOx in the exhaust gas 3 are lessened.
In this case, the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is reversely curved back by the communication passage 9 into the entry end of the adjacent selective reduction catalyst 6. As a result, enough reaction time is ensured for production of ammonia from the urea water since a long distance between the selective reduction catalyst 6 and the urea water added position by the urea water addition means 10 is ensured and the flow of the exhaust gas 3 becomes turbulent due to the reversed curving to facilitate mixing of the urea water with the exhaust gas 3.
Moreover, the particulate filter 5 and selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged between and along the filter 5 and catalyst 6, so that the whole structure becomes compact in size to substantially improve its mountability on a vehicle.
However, with such structure having the heavy particulate filter 5 and heavy selective reduction catalyst 6 arranged side by side and interconnected by the S-shaped communication passage 9, relative twisting tends to be caused between the filter 5 and the catalyst 6, so that stress may be concentrated to connections and curved portions of the communication passage 9. In order to overcome this, as shown in FIG. 2, the casings 7 and 8 for the particulate filter 5 and selective reduction catalyst 6 are interconnected at plural axial positions through connections 13 and 14 on upper and lower sides so as not to be relatively twisted. Then, the casings 7 and 8 are arranged between a pair of brackets 16 and 17 fixed to a frame 15 on a vehicle body and are supported through connections 18 and 19 by the brackets 16 and 17, respectively.
As a prior art literature pertinent to the invention, there exists, for example, the following Patent Literature 1.                [Patent Literature 1] JP 2005-155404A        